The invention relates generally to monitoring groundwater contamination and, more particularly, to a vadose zone monitoring system using horizontal access tubes and neutron moderation techniques for detecting leakage from waste management units ("WMU's") such as landfills or other sources of contaminants.
Before contaminants can enter the groundwater system they must first pass through the vadose zone, a complex intermediate zone between the surface and the permanent groundwater table. A relatively dry vadose zone acts like a sponge, holding liquids in pore and crack spaces. Therefore, the vadose zone is an important roadblock to contaminant transport in the subsurface and serves as an ideal milieux for the containment and isolation of contaminants until they can be rendered harmless or decompose. Outward flow of contaminants from the vadose zone can be eliminated by stopping the contaminant sources and surface infiltration because liquid mobility decreases at lower saturation percentage. Effective vadose zone monitoring, which detects contaminants and/or progressive saturation before groundwater is impacted, focuses efforts on contamination reduction rather than remediation.
Currently employed vadose zone monitoring systems provide incomplete assessments of the vadose zone. The limitations and drawbacks of commonly employed vadose zone monitoring systems are discussed below.
Direct pore liquid sampling by vacuum lysimetry is by far the most commonly employed vadose zone monitoring strategy. This and other techniques of pore-liquid sampling have been thoroughly described in the literature. Lysimeters provide direct chemical confirmation of the presence or absence of contaminants if soil tensions are in their effective operating range (0-65 char), and indirect evidence if not. If the soils are too dry to collect a lysimeter sample, they are also too dry for contaminant migration. The most severe limitation of this method is the sampling radius. The sampling radius is dependent upon the surrounding soil system, its moisture content and the time between sampling events. Previous work by others indicates network spacing of 7 to 17 feet for a variety of appropriate soils and leak rates.
Comprehensive lysimeter network design is rarely achieved in practice because it requires a large number of sampler installations, which is prohibitively expensive. For example, complete coverage of a forty acre site would require 6,000 to 35,000 samplers. Aside from installation costs, the annual costs for chemical analyses to satisfy a typical monitoring order in the state of California would be $20 million. In practice, a typical lysimeter network for a forty acre landfill might have a total of five samplers at depths of 2 to 100 meters. Liquid sampling systems therefor represent spot checks of liquid movement in the vadose zone.
The importance of moisture front detection is apparent from FIG. 1 which shows a plot of unsaturated hydraulic conductivity (K.sub.u) of a clay material as a function of moisture content. Note that an exponential rise in K.sub.u occurs throughout the upper 50% of saturation percentage. A typical clay liner is installed at approximately 30% volumetric water content (water content by wt. of approximately 16%). At these water contents K.sub.u is orders of magnitude less than the already low saturated hydraulic conductivity (K.sub.s) for this material. Monitoring soil moisture through this interval identifies when K.sub.u is increasing rapidly, which is also when contaminants become mobile.
The same logic applies to poorly graded coarse material with a distinct air entry pressure, such as sand. Increases in K.sub.u in these materials is accompanied by large increases in water content. Unfortunately, they also drain very rapidly and unless measuring events are continuous, frequent, or timed to coincide with probable leak events, e.g., after heavy rains, leaks may pass undetected through these materials. Most soils are texturally intermediate in which moisture changes in response to leak events will be more gradual than sands.
The construction of new WMUs has provided opportunities to proact to environmental problems by planning for vadose zone monitoring at the inception of new facility design. Most sampling strategies employ the concepts of compliance points which must be periodically monitored for specified contaminants. Compliance points, originally conceived for groundwater monitoring, usually define a vertical plane down gradient from the WMU. The predominately vertical flow in the vadose zone makes it preferable to choose points directly beneath the facilities which define a horizontal plane. This is rarely done unless the vadose zone monitoring system can be planned during construction of the facility.
The strategic placement of horizontal access tubes, laid down like pipe beneath the WMU before construction, permits indirect pore liquid monitoring through geophysical techniques such as neutron moderation logging discussed in detail below. The resulting logs are line samples, a considerable improvement in coverage over the point sampling strategy described above. Logs of moisture content along these transects detect and locate potential leachate leaks, directly beneath the WMU. (Example logs are shown in FIGS. 3-4 discussed below). Chemical confirmation can be obtained from soil gas or pore liquid samplers retrofitted to specific problem areas. This results in a more judicious use of liquid samplers and chemical analyses than otherwise possible. Thus space and time coverage can be enhanced by monitoring the pore liquid content beneath the facility rather than just the chemistry of pore liquids.
Despite the improved coverage provided by horizontal access tubes, comprehensive coverage beneath WMUs has not yet been attained by these techniques. Borehole geophysical logging has a limited radius of investigation, usually less than 70 cm, although experimental techniques such as cross hole resistivity are under investigation to extend this range. For example, the neutron probe system described below measures a maximum cylinder of radius approximately 65 cm in dry sandy material and much less in wetter soils. Complete coverage under optimum homogeneous conditions would require a tight network of tubes 130 cm on center or less. This corresponds to providing tubes spaced no more than 260 cm apart. As with the lysimeter network design, complete coverage of a WMU using horizontal tubes is commercially unfeasible due to exorbitant cost.
Neutron moderation logging has been used to indicate the moisture content of soil surrounding the horizontal access tubes through the use of a neutron probe movably disposed inside the tube. Neutron moisture logging uses the neutron moderation technique, in which fast neutrons emitted from a neutron source in the probe, such as Americium 241/Beryllium through the tube and into the test material, which comprises the soil surrounding the tube, where the neutrons collide elastically with atoms. A detector also disposed in the probe is responsive to slow or thermalized neutrons only. The mass of most elements greatly exceeds the mass of fast neutrons so that these collisions result in little measurable loss of momentum from the fast neutron. It takes hundreds of such collisions to decelerate a fast neutron to its slow or thermalized state. Hydrogen, however, has a mass identical to that of the fast neutron, which means that collisions with hydrogen will result in a significant transfer of velocity from the fast neutron. It takes only about 20 such collisions to thermalize a fast neutron. The detector counts the number of thermalized neutrons it receives over a given time period. These counts can be correlated directly with hydrogen concentrations since the probability that the thermalized neutrons are the result of collisions with hydrogen is much greater than the probability that they are from other collisions. This is a measure of pore liquid volume, since the most common source of hydrogen in the geologic environment is pore liquids.
Commercially available moisture probes measure the hydrogen density in a spherical volume around the probe which can be integrated along an access tube for a cylindrical sample of radius 15 to 65 cm. This translates to real advantages over direct pore liquid sampling, which requires periodic sampling, usually quarterly, with delay time between sample collection and analytical results, point samples with limited lateral coverage, and large sampling budgets.
Neutron moderation is a well documented technology developed for measuring the volumetric moisture content of soils. It has been used to track wetting fronts in the vadose zone and recently has been applied to vadose zone monitoring at WMUs. Testing at a hazardous waste facility clearly has shown elevated neutron counts in response to a simulated leak. It has been a proven technique to detect hydrocarbon liquids, as well as water, and has therefore been proposed as an important component of new multiplexing monitoring strategies. The applicability of the technique to numerous groundwater monitoring wells, not originally designed for vadose zone monitoring, has been demonstrated in the laboratory where the masking effects of grouts and casings did not preclude the detection of wetting fronts in and regimes. This permits the use of neutron logging in piezometers and monitoring wells to track infiltration and map perched water horizons, which may produce false positives in horizontal access tubes beneath facilities.
Horizontal access tubes have been applied to several sites to provide vadose zone monitoring systems at the inception of a new facility. One such a site is for waste water ponds located in the Mojave Desert area. Ten horizontal access tubes 263 meters long were used in conjunction with vertical well logging to obtain a three dimensional picture of the moisture content beneath the ponds. A second application is at a new Class I California landfill where four horizontal access tubes (213 meters) were installed beneath the leachate sump during construction. Example logs from this site are shown in FIG. 4. At these sites logging is performed by manually deploying the probes through the tubes on a regular schedule, which requires significant man hours.
Calaveras County, California has constructed a new landfill facility that incorporates a vadose zone monitoring system having a neutron access tube disposed beneath the swale, leachate pipeline and pond as shown in FIGS. 2a, 2b. The neutron monitoring system includes an access tube of 4" inner diameter, high density polyethylene (HDPE) buried a foot below the overbuilt clay liner beneath the leachate recovery system as shown in FIG. 2b. The access tube was graded to drain to avoid ponding of condensate or other water, and perforated to allow the entrance of soil gas. The neutron probe used was manually operable and adapted to transmit its measurements over the full length of deployed cable. The cable contained several conductors and a slip ring connector at the spool. Initial neutron logs showing the results of monitoring events at this site are shown in FIG. 3.
The disadvantages of neutron probe monitoring have been discussed in the literature and include a limited cylinder of investigation (typical radius of 5-64 cm), measurement of only volumetric changes in pore fluid and therefore insensitivity to steady state flow, and lack of chemical data. The foregoing review indicates that the need for a commercially feasible, vadose zone monitoring system for detecting contaminant leaks from substantially all areas of a WMU has not been met. No such designs for complete coverage of an entire landfill exist at this time. The systems in use place the horizontal access tubes beneath areas likely to pond leachate, such as sumps and leachate collection systems.